'A cross between a Billingsgate fishwife and Lucretia Borgia." That was how the fashion designer Alexander McQueen once described Isabella Blow, at various times fashion supremo of Tatler, Vogue and the Sunday Times, who has died in hospital aged 48 after suffering from cancer and depression.

She was most famous as a talent-spotter and benefactor to young British designers. She championed McQueen after buying his entire graduate collection in 1992, and discovered and nurtured the milliner Philip Treacy, whose elaborate hats became her trademark. She also discovered the models Sophie Dahl and Stella Tennant.

From 1997 to 2001, she was fashion director of the Sunday Times Style magazine, where I was her assistant. Her distinctive tuberose scent heralded her arrival at News International's drab offices in east London, if you didn't spot one of her Treacy hats on the horizon first. This was the signal to apply lipstick and high heels - according to Isabella, wearing jeans and trainers was a sackable offence. Often, because of the rush, lipstick would end up on my teeth or halfway down my chin, but this was fine because that was Isabella's look, too.

When her expenses became too inflated, she agreed to use the underground instead of taxis, and found that she enjoyed it. I would meet her at the station to show her the back route to the office - it was a daily worry that the sight of her, in stilettos, furs, perhaps her one-legged trouser suit and hat, would cause a road accident on the main dual carriageway.

Isabella - or Izzie, as she was known - said that her love of fashion came from her grandmother, Lady Vera Delves Broughton, a photographer, explorer and hunter. The family had lived at Doddington, a castle with 35,000 acres of land in Cheshire, since the 14th century, but it was sold to pay off her grandfather's gambling debts and, as a child, Isabella could see the castle only from her family's cottage on the estate. Her parents never got over the death of their only son - a two-year-old who Isabella saw drown in the family swimming pool - and she told me they had seemed to lose interest in her and her sisters.

Born in London, she was sent to Heathfield school, in Ascot, Surrey. When she was 14, her mother announced she was leaving the family, shook her hand and said goodbye; Isabella rarely saw her after that. Her father, Sir Evelyn Delves Broughton, remarried. When he died in 1993, he left her £5,000 of his £6m fortune. She was never good at holding on to money, despite lucrative advertising deals and consultancies (notably with Swarovski; she was the driving force behind the reinvention of the crystal company), because her generosity was enormous.

After A-levels and a secretarial course, she took odd jobs, such as cleaning. Even then, she would fashion a dishcloth into an elaborate hat to keep her hair out of her face. She moved to New York in 1979 to study Chinese art at Columbia University. In 1981, she married Nicholas Taylor, but they divorced two years later. Her friend the musician Bryan Ferry introduced her to Anna Wintour, editor of American Vogue, who hired her as her assistant.

She became part of the avant-garde New York scene and was friends with Andy Warhol, Roy Lichtenstein and Jean-Michel Basquiat. She came back to London in 1986 as assistant to the then Tatler fashion director, Michael Roberts. In 1993 she went to British Vogue, and after the Sunday Times returned to Tatler as fashion director.

In 1989, she met her second husband, Detmar Blow, a lawyer and later an art dealer. When they married at Gloucester Cathedral, she wore a headdress commissioned by the then-unknown Treacy, which marked the start of their friendship.

Isabella's appearance - Wallis Simpson as envisaged by Salvador Dalí - at the front row of fashion shows became as eagerly awaited as the collections themselves. She once wore a jewel-encrusted lobster on her head, and on another occasion an outfit, inspired by Joan of Arc, which included a heavy, oily chain that she dragged behind her. Afterwards, she visited Karl Lagerfeld at his Paris home and dragged the dirty chain all over his plush cream carpets.

At a lunch with Nicholas Coleridge, managing director of Condé Nast, she wore a pair of antlers covered in a heavy black lace veil. When he asked how she would able to eat, she said: "Nicholas, that is of no concern to me whatsoever."

Towards the end of her life, Isabella had become as recognisable as the designers and young artists she championed. The Design Museum held an exhibition in 2002 entitled When Philip Met Isabella, celebrating the relationship between Treacy and his muse. In 2004, she had a cameo appearance in the film The Life Aquatic With Steve Zissou.

She is survived by Detmar and a considerable hat collection.

Geordie Greig writes: Izzie Blow never did dull. She was an original who always dealt in surprises. For 25 years I knew her, and particularly closely for the last six at Tatler. For every meeting in my office she wore a hat, was always immaculately made-up and wearing, more often than not, a dress more suited to a ball than corporate life, but she was never a cliched fashionista. "Sorry, Geordie, I forgot my knickers," she would say, knowingly. "Oh sorry," she would burst out laughing, suddenly realising a breast had fallen out of her top.

She loved over the top. What she always did in her work was to source a detail and turn it into art. And art was what fuelled her and what she created. She created images in magazines as compelling as Dalí did in paint.

People would ask if she was scary. The answer was she was the friendliest, most generous, exciting, impossible, inventive, life-enhancing wonderful woman in fashion. Her ambition was limitless, her canvas global, her influences minor and monumental, her contacts unpredictable. She was fabulously grand ("I have five maharaja's daughters to shoot") and sometimes grandiose ("Do you want to come boar-hunting in northern Kenya?") but she was always compelling as well as inclusive.

She was elitist and selective. But she was no snob. She loved people from anywhere as long as they had a view and a voice. In an NHS hospital in outer London, she shared a ward with an energised East Ender. Friendship blossomed as Isabella shared all she had: her life, her views, her drama, her pain - and her passion for life. Few people are ever a real diamond - Izzie Blow was.

· Isabella Blow, fashion journalist, born November 19 1958; died May 7 2007

The late Isabella Blow, influential fashion stylist and champion of the avant garde, launched the careers of many of fashion's household names, including the model Sophie Dahl, designer Alexander McQueen and milliner Philip Treacy. However, she remains best known for her extravagant hats. Here we celebrate her finest moments...